1. Field of the Invention
The present invention relates to fluid pressure operated brake systems and more particularly to an apparatus for actuating an emergency brake on loss of fluid pressure in the brake system.
2. Description of the Prior Art
Emergency brake actuators which are responsive to pressure in a brake system are used in various types of devices such as turbines, off-road vehicles and the like to prevent damage in the event of loss of system pressure. Generally these actuators include a single large compression spring which is housed within one end of a cylindrical housing to actuate the emergency brake on loss of system pressure. The other end of the cylindrical housing is used as a fluid pressure chamber for compressing the spring to release the emergency brake when the brake system is pressurized. The actuator must provide an instantaneous response to a loss of pressure in the system in order to prevent damage to the wind turbine, off-road vehicle or the like.
The compression spring is initially partially compressed within the housing to hold the emergency brake in the applied position. When the fluid system is pressurized, the pressure chamber is filled with hydraulic fluid to fully compress the spring and release the emergency brake. When fully compressed the spring must have sufficient force to not only actuate the emergency brake but to also discharge the pressurized fluid from the chamber. This can cause a momentary delay in applying the brake due to the large amount of fluid in the pressure chamber. Special care is necessary in handling and repairing these actuators since the spring force of the precompressed spring is sufficient to cause an explosion on opening the housing. Extreme caution is therefore required in order to avoid accidents on opening of the spring housing.
If a single high compression spring is used to provide the force necessary to operate the actuator, the spring force of the spring can deteriorate over time due to the high stress on the spring when fully compressed. A high compression spring can lose from 10% to 21% of its force capability if held under compression over a long period of time.